This invention relates to dual-phase steel, and notably to procedure for making it, as well as to novel hot-rolled and cold-rolled products so manufactured, e.g. in the form of annealed strip. The invention also especially relates to steel tubing having a welded seam and made from skelp of annealed steel of dual-phase character. One particularly important aspect of the invention resides in procedure whereby cold-rolled, annealed skelp of dual-phase steel is produced and is converted to steel tubing, for instance tubing of such dimensional and manipulable nature that it can be rolled up into a coil for storage and transportation, as in the course of use or re-use for so-called down-well service in oil well operations. Another important aspect of the invention in the area of tubing is in respect to the use of hot-rolled, annealed skelp of dual-phase steel for making casing, such as oil well casing having outside diameter in the range of 6 to 15 inches or similar pipe from smaller to larger diameters.
Steel of a type that has come to be called dual-phase, usually produced in strip or equivalent form, has an internal structure characterized by islands, which are more or less discrete bodies (although sometimes interconnected) of primarily martensitic character, surrounded by a primarily ferritic matrix. This structure, thus consisting of a first or matrix phase which is essentially ferrite and which represents 70% or more by volume of the product, together with a second or distributed phase of contained bodies that constitute 10% or more of the product by volume and each have a major content of martensite, is understood to result from selected composition and processing and to afford certain advantageous mechanical properties now recognized as characterizing dual-phase steel. These properties in general involve superior formability and superior strength, especially relative to the strength:weight ratio of the steel and relative to its cost in the area of alloying elements; stated in another way, a prime advantage of dual-phase steels is in their ready formability (as to drawing, stretching, bending and the like), while exhibiting exceptional strength, in a manner not generally attained by presently conventional high-strength low-alloy (HSLA) steels.
In its as-produced form dual-phase steel is characterized by high ultimate tensile strength, as in the range of 70 ksi to 100 ksi or above, and relatively low initial yield strength, such that the ratio of yield to ultimate strength is of the order of not more than about 0.65 (preferably 0.6 or less). At the same time, the steel has high elongation (e.g. total elongation of 20% or up to 30% or so). The steel on deformation exhibits continuous yielding (homogeneous deformation rather than discontinuous) and rapid strain hardening which results in relatively high values of uniform elongation. This uniform elongation is preferably at least 15% or 18% or higher. The rate of strain hardening can advantageously be relatively great, so that on forming, as with moderate deformation, the yield strength rises quickly to relatively high values, as by increasing from 60 ksi to 80 ksi. These features, including the low initial yield strength and large, uniform elongation, make the steel relatively easy to form, with low forming loads, and less springback, while achieving advantageously high strength levels in the shaped products. As explained, the steel preferably has a very high rate of strain hardening, for instance so that the formed part can develop an 80 ksi flow stress after only 3 to 5% strain. The index of stretching formability, called "n", is relatively high for dual-phase steels, being greater than 0.2 in contrast to n-values of 0.1 to 0.13 ordinarily found for HSLA steels. It has been observed that n is not only a measure of the ability of the steel to resist necking, but if high, is also significant of more uniform redistribution of strain in a steel when thinning does occur.
An informative discussion of dual-phase steel and its recent state of development appears in W. S. Owen, Can a Simple Heat Treatment Help to Save Detroit? (Fifth Harold Moore Lecture), Metals Technology, Jan. 1980, pp. 1-13.
In general, dual-phase steels have been produced by employing a selected elemental composition or chemistry, and by following a selected processing technique, e.g. involving a so-called intercritical anneal or its equivalent, to achieve the desired structure and properties, in cold-rolled or hot-rolled products. One process has been to subject the hot-rolled or cold-rolled strip to continuous annealing, i.e. in a long, closed passage where the steel is heated rapidly to a temperature within the alpha plus gamma, or gamma region for a matter of minutes and then cooled relatively rapidly at rates corresponding to air cooling up to that of water quenching. Such air cooling can yield a cooling rate of around 20.degree. F./second (7.2.times.10.sup.4 F/hour). This technique can be used to produce both hot and cold rolled thicknesses, but is usually limited to cold rolled thicknesses to about 0.060 inch or light hot bands, 0.065 to 0.150 inch, depending on furnace design.
By a carefully controlled hot rolling operation, dualphase steel can be produced, but only in hot band gages. Thus the steel is rolled in a controlled manner such that the required amount of ferrite is formed during finish rolling, or on the run-out table before quenching (or at both localities, together), and thereafter the desired volume fraction of second phase constituent is achieved by water quenching the remaining austenite phase on the run-out table. Thus on water quenching the strip, the occurring bodies of austenite undergo transformation to martensite, with perhaps some bainite and some retained austenite. This transformation may occur during the quench or during cooling of the coil, depending on the alloy and on coiling temperature. This technique, limited to hot rolled product and requiring difficult control, is not commercially very attractive, while the continuous anneal requires costly equipment, not possessed by many steel producers.
Making dual-phase steel by a batch anneal, where coils of the original rolled strip (either H-R or C-R) are stacked in conventional annealing furnaces, and there subjected to heating and cooling in succession, would seem inherently convenient and economical, except that the heating and cooling cycle of a continuous anneal is a matter of minutes, presumably facilitating precise attainment of the desired transformations or conversions, whereas the cycle of a batch anneal will typically take two or more days. Indeed, reported experience of others in attempting to make dual-phase steel by batch annealing, even with inclusion of selected additions, such as small quantities of Cb and/or Ti, or V has not reached the desired results of both formability and strength, and such other workers have resorted to very high levels of Mn (e.g. 3%), which lead to steelmaking difficulties, in order to achieve the desired results. Experiments leading to the present invention revealed similar difficulty in attaining success by a batch anneal technique.